Two counties are reporting just one case each, while six counties have multiple cases. King County tops the list at 176 confirmed or probable cases. Spokane County has 165, Tacoma-Pierce County has 45, Grant County has nine, Snohomish County has four, Ferry County has three and Thurston and Yakima Counties each just one.

Approximately 25% of the total infections occurred among people within the 14 to 18 age group, 22% in the 10 to 13 age group, 14% in the 5 to 9 age group, 5% in the 19 to 24 age group and 3% in the infant to 4 age group. People over 25 made up the remaining reported infections. No deaths have been reported.

"It's been a continuous upwards track of new cases. However, until we reach a point where no more vulnerable people are exposed, it may continue to grow," said Dave Johnson, a spokesman for the Washington State Department of Health.

According to Kim Papich, spokeswoman for the Spokane Regional Health District, "what we're seeing is expected, especially as we look to other areas that are dealing with outbreaks, like King County." The outbreaks among "partners in Arkansas and other areas of the country" also indicate that the outbreak in Washington is continuing in a typical manner.

Each year, the number of mumps cases fluctuates within a range of a couple hundred to a couple thousand cases, according to Dr. Manisha Patel, a medical officer at the US Centers for Disease Control and Prevention. During 2016, a "high" year, 46 states and the District of Columbia reported 5,311 total cases but only a handful of states pushed the numbers above average. Arkansas, Iowa, Oklahoma, New York and Illinois each confirmed more than 300 cases.

"Just looking at incubation period and method of transmission, we expect that we're going to still go through a few more waves of the outbreak, so we don't expect cases to necessarily slow down yet," Papich said.

According to Johnson, the department of health is "assisting local health departments with the case investigations" and giving "clinical guidance to health care providers about diagnosing and testing for mumps."

"We continue to supplying vaccine to local health departments to be sure there is enough for people who need it," he said. "The best way to control the mumps outbreak is to educate people about how to protect themselves and their families from mumps. The MMR vaccine is our first line of defense."

Before the US mumps vaccination program started in 1967, the CDC received reports of 186,000 cases each year. Since most doctors considered the mumps a typical childhood disease, the actual number of cases was probably much higher. Cases decreased by more than 99% in the US in the years after the vaccination program initially began.

Lessons learned in Arkansas

According to Dr. Dirk Haselow, Arkansas' state epidemiologist, an outbreak of mumps began there in August. There have been 2,766 cases since then.

"At its height, we were seeing 50 cases a day," he said, but after Thanksgiving and Christmas, the numbers began to drop "dramatically."

Though he doesn't have a crystal ball, Haselow said, "based on the way the epidemic curve is going down, if it continues on that trend, we may be lucky enough to be out of this in a couple of months." A mumps outbreak is over, he explained, when a state has a period of a full 26 days without any cases, the longest potential incubation period.

"This is the second-largest outbreak of mumps in the US in the last 25 years, and it is by far the most diverse outbreak," he said. The biggest modern outbreak occurred in a group of Orthodox boys in a boarding school in New York, a limited setting, he said. "Ours is over a hundred schools, 30 employment centers, five colleges -- it's very widespread, and because of that, we can learn a lot."

Like 15 other states, Arkansas allows parents to opt out of vaccinations for their children based on medical, religious or philosophical exemptions or reasons.

"I don't think this outbreak is driven by exemptions, although I'm not fond of exemptions, and I don't want to discount how important they can be," Haselow said. "Compared to the nation, we are indeed a little bit lower (on vaccination coverage)."

Entering the 2015-16 school year, national MMR vaccine coverage for children enrolled in kindergarten ranged from 87.1% in Colorado to 99.4% in Maryland, with Arkansas at 90.8% coverage and Washington at 91%, according to the CDC. Among teens, national MMR coverage during 2015 ranged from 80.4% (Idaho) to 97.4% (Rhode Island) with Arkansas at 90.2% coverage and Washington at 88.1%.

"Our outbreak is much more explained by crowding and poverty than it is by under-vaccination," Haselow said.

According to Johnson, in Washington state, 91% of kids and teens who caught the mumps (and had a known vaccination status) had the recommended two MMR shots.

Although getting a vaccine may not prevent illness, it probably helps those who do get sick avoid serious complications.

"During the pre-vaccine era, between 20% and 50% of boys would develop orchitis: swollen testicles," Haselow said. "Of those, a third would develop infertility. Arkansas has seen only 17 boys develop orchitis in this outbreak, much fewer than in the pre-vaccination era."

Arkansas has not seen any cases of meningitis (swelling of the protective membranes covering the brain and spinal cord), pancreatitis (inflammation of the pancreas, a gland that helps process sugar) or encephalitis (acute swelling of the brain).

"We've only had six hospitalizations," Haselow said. "So the vaccine is definitely making the illness milder than it would otherwise be."

Still, having also seen outbreaks in 2006 and 2009, he's concerned about the unexplained "resurgence" of mumps.

"Initially, we were concerned this strain was a unique strain that was in some way evading the vaccine," he said. "We've had more than a dozen samples from our outbreak sequenced and determined that, no, it's not an unusual strain, it's the same strain the United States had for 30 years and the same strain involved in all these other outbreaks.

"So why here? Why now? To some extent, bad luck; to some extent, we think it's related to crowding," he said.

How to halt an outbreak

Officials urge people to get vaccinated and to take precautions to stop the spread.

The CDC recommends that children get two doses of the MMR vaccine, which protects against measles, mumps and rubella, though it is not 100% effective. The CDC is working closely with the state department of health, according to Patel.

If you suspect someone of having mumps, you should avoid kissing, hugging and other close contact. Anyone feeling sick should contact their health care provider, local health departments or the Family Health Hotline at 1-800-322-2588, Washington health authorities said.

Mumps typically begins with fever, headache, muscle aches, tiredness and loss of appetite lasting a few days. Most people will then see salivary glands swell, causing puffy cheeks and a swollen jaw. It can take 12 to 25 days to become ill after exposure to the virus. Once symptoms begin they can last from two to 10 days.

Mumps can occasionally cause complications, especially in adults. These include hearing loss and inflammation of the brain, ovaries, breast tissue or testicles. Occasionally, mumps can cause encephalitis, which in rare cases can lead to death, says the CDC.

Because it is caused by a virus, mumps will not respond to antibiotics. Doctors generally recommend bed rest and over-the-counter pain relievers.

Dosage debate

The CDC recommends that children get their first dose of MMR vaccine at 12 to 15 months of age and the second dose at 4 to 6 years old. The alternative MMVR vaccine, which protects against measles, mumps, rubella and varicella (chickenpox), is licensed for use in children between the ages of 12 months and 12 years.

In people who receive just one dose, the vaccine is less successful at preventing illness than the recommended two doses, which provide 88% effectiveness.

As calculated by scientists, 88% effectiveness does not mean that of 100 people coming into contact with the mumps virus, only 12 people will get sick. It is a more complex estimation because some people will be repeatedly exposed to the virus -- such as children, nurses and teachers -- so how often you come into contact with the virus also plays a role in whether you might become sick, even when vaccinated.

Outbreaks in recent years, most notably one involving NHL players in 2014, have spurred debate over the need for a third dose of vaccine.

The Advisory Committee on Immunization Practices -- a panel of medical and public health experts who meet three times a year to offer vaccination guidance for the United States -- has begun the work to consider the recommendation of a third dose of vaccine.

A working group will consider multiple components and data from ongoing effectiveness studies and then present its recommendations to the committee for a vote, Patel said.

Arkansas, which tested the use of a third dose in some regions within the state outbreak, will be providing some of this necessary data, according to Haselow. "A recent dose of the MMR vaccine is potentially very helpful in the control of outbreaks."

According to Patel, "safety, effectiveness, implementation -- a number of things are considered when a recommendation is put forth," so the overall process of deciding on a third dose will probably take 12 to 18 months.